Exemplary embodiments of the invention relate to a space-borne antenna system, comprising a number or panels being moveable to each other and having a gap in between them when the panels are arranged in an operation condition. The antenna system further comprises a RF distribution network for providing transmit signals to the number of panels and combining received signals from the number of panels and a set of choke flange assemblies which allow a contactless inter-panel signal transmission across a dedicated gap, wherein a respective choke flange assembly is arranged on the far side of a radiating surface of the dedicated adjacent panels.
Antenna systems for space applications are deployed in space while they are folded for transportation. After having deployed the antenna system it is necessary to couple adjacent panels of the antenna system for signal transmission.
An antenna system of the type above is, for example, the Sentinel-1 SAR Antenna Subsystem (SAS) for the Sentinel-1 mission. This antenna system is a deployable planar active phased array antenna working in C-band (5.405 GHz) with a frequency bandwidth of 100 MHz. The antenna has an overall size of 12.3 m×0.84 m and is formed by a central panel mounted on top of the spacecraft and two antenna side wings at the two adjacent sides of the spacecraft. The central panel is equipped with two SAS tiles, whereas the two panels of each side wing carry three SAS tiles each. This leads to an overall number of 14 identical tiles: 6 (SAS right wing)+2 (SAS central panel)+6 (SAS left wing). Each SAS tile possesses all the functions needed to allow for beam shaping and steering.
Generally, the SAS encompasses the following principal functionalities: signal radiation and reception (WG-Assy); distributed transmit signal high power amplification (EFEs, TAAs); distributed receive signal low noise amplification with LNA protection (EFEs); signal and power distribution (corporate feed, power converter) (RFDN); phase and amplitude control including temperature compensation (EFEs via TCU); internal calibration loop; deployment mechanisms including hold down and release; and antenna mechanical structure.
Regarding the RF-signal power distribution, on panel level the Sentinel-1 SAR Instrument RF-Distribution Network (RFDN) distributes in TX the signals from the SAR Electronic Subsystem (SES) to the antenna tiles (i.e. to the input port of the Tile Amplifier Assembly (TAA)) with a good phase match. On SAS tile level the RFDN distributes the TX signals from the output of the tile amplifier assembly to the Electronic Front End (EFE) modules with a good phase match. For RX the RFDN combines the received signal in the reverse direction.
The RF-Distribution Network is made up of the following elements:                the Azimuth Plane Distribution Network (APDN), for panel level signal distribution        the Elevation Plane Distribution Network (EPDN), for SAS tile level signal distribution        the RF harness        
In summary, the RFDN possesses the following major functions:                For TX: Distribute the TX signal from the SES via the tile amplifiers to the EFEs with a small phase variation between the output ports.        For RX: Combine the received signal from the EFEs via the tile amplifiers towards the SES with a small phase variation between the different RX paths.        Band pass filtering in the TX and RX path.        
On tile level, the EPDN of the RFDN consists of coaxial cables and power dividers/combiners. On panel level, the APDN encompasses coaxial cables and power divider/combiner composite as well. For the Inter-Panel RF Harness routing, connection of the three RF harness branches (TX, RX-V and RX-H) from panel to panel after deployment is achieved by a set of dedicated choke flange connections, which allow a contactless inter-panel signal transmission. The choke flange assemblies are located in the center of the Antenna Panel Frame (APF) transverse beam.
It has been found in tests that a high amplitude ripple in transmit calibration mode (TX Cal) occurs for horizontal polarized signals. This makes it difficult to conduct an internal calibration.
Accordingly, exemplary embodiments of the present invention are directed to an antenna system in which an internal calibration can be made easier and more reliable.
In order to improve internal calibration, a space-borne antenna system is disclosed, which comprises a number or panels being moveable to each other and having a gap in between them when the panels are arranged in an operation condition; an RF distribution network for providing transmit signals to the number of panels and combining received signals from the number of panels; and a set of choke flange assemblies which allow a contactless inter-panel signal transmission across a dedicated gap, wherein a respective choke flange assembly is arranged on the far side of a radiating surface of the dedicated adjacent panels. Furthermore, the antenna system comprises an RF (radio frequency) seal assembly for suppressing a signal coupling of signals radiated from the number of panels to the set of choke flange assemblies by sealing the gap.
The invention is based on the consideration that the high amplitude ripple in transmit mode that occurs for horizontal polarized signals is a result of coupling from the antenna waveguide radiators to the choke flange assembly between two panels. An RF seal is added to the junction between two adjacent panels to minimize the coupling from waveguide radiators to the choke flange assembly. The added seal is made such that it does not counter-act to a panel latching mechanism. Hence, the RF seal is provided in a way to not exert excessive additional mechanical force while it does not require mechanical contact between the panels. As a result, the RF seal assembly closes the gaps between panels, specifically tiles between two adjacent panels.
According to a further embodiment a respective RF seal assembly is dedicated to a gap between two adjacent panels of the number of panels.
A respective RF seal assembly may comprise a first and a second seal profile that are affixed in opposing pairs in the gap between two adjacent panels of the number of panels. The profiles enable closing the gap between the panels, specifically tiles within the panels.
The first and the second seal profile may have an L-shaped cross-section, in a side view in a longitudinal section through the antenna system. First portions of the first and the second seal profile extend in a plane of the number of panels, when the panels are arranged in an operation condition, and are attached to the dedicated adjacent panel. Second portions of the first and the second seal profile extend in a direction of radiation of signals such that they are opposing and having a gap in between them. This shape, on the one hand, enables closing the gap between the panels. On the other hand, it does not count to a panel latching mechanism.
In one embodiment, the gap between the second portions of the first and the second seal profile has a constant width in a direction of radiation of signals. In this configuration, the second seal profiles are perpendicular to the plane of the panels, when the panels are arranged in an operation condition, i.e., the angle between the first and the second portion of a respective profile is 90°.
In an alternative embodiment, the gap between the second portions of the first and the second seal profile has a widening or a narrowing width in a direction of radiation of signals, resulting in an angle between the first and the second portion of a respective profile which is less or r more than 90°.
It is preferred that the RF seal assembly is made from the material of radiating waveguides of the set of panels. This ensures that the RF seal assembly and the waveguides have the same coefficients of thermal expansion resulting in minimized therm-mechanical stress. The profiles of the RF seal assembly may be made from CFRP, in particular metallized CFRP. CFRP is a Carbon fiber reinforced plastic. This allows manufacturing the profiles from left-over antenna waveguides. Alternatively, the RF seal assembly may be made from a metal, e.g. aluminum.
In a further preferred embodiment, the RF seal assembly is mechanically attached to the adjacent panels via at least one adhesive tape, in particular a high adhesive double sided tape. As one of the adhesive tapes, for example, 3M #Y966 tape may be used. Such kind of tape is used for heavy duty hold down applications where a high level of adhesion is required.
In a further preferred embodiment, the RF seal assembly is electrically coupled to the adjacent panels via a metal adhesive tape. The metal adhesive tape may be, for example, a Cho-foil, which has good shielding and conductivity properties with respect to EMI (Electro-magnetic Interference). This assists suppressing the signal coupling of signals radiated form the number of panels to the set of choke flange assemblies
According to a further preferred embodiment, the RF seal assembly is arranged at a hinge line of the antenna system.
The RF seal assembly can be regarded as a choke configuration that is used to close the gaps between panels, i.e. tiles within the panels.
In the figures, like elements are depicted with like reference numerals. It is to be noted that the embodiments shown in the figures are not drawn to scale and are used to illustrate the basic concept of the invention.